Slew rate and output impedance are often competing parameters in an IO driver circuit design. The slew rate refers to the maximum rate of change for the output voltage of a circuit for all possible input signals. From the perspective of an IO driver that is driven by a binary input signal, the slew rate effectively relates to how quickly the driver can switch the output of the driver between two logic states.
Output impedance generally refers to the characteristic impedance of the output of a circuit. Control over output impedance is typically required for the purpose of impedance matching with a load. When the output impedance of a circuit is mismatched with that of its load, the fidelity of the signal being transmitted can degrade due to reflections and distortions of the signal.
A requirement for high driver output impedance generally introduces delays into a circuit, and as such, often has the net effect of decreasing the slew rate of the circuit. As circuits such as IO drivers are used in increasingly faster and higher performing circuits, however, the need to increase the slew rates of such drivers has increased. Many specifications, on the other hand, require a specific output impedance, and as such, the output impedance cannot simply be scaled down to assist with improving the slew rate of a driver.
To reconcile these conflicting requirements, a need exists in the art for a manner of improving the slew rate of an IO driver while maintaining a desirable output impedance for the driver.